Silk and concrete: Old materials and new methods could revolutionize construction

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In his classic 1998 work, Design in Nature – Learning from Trees, Claus Mattheck presented a heretical new idea — removing material in the right places can actually make things stronger. By introducing smooth transitions to regions where angular or Cartesian geometry channeled stress into weak points, he could delay the onset of crack formation. Now, two bold new ideas, one using concrete, the other silk, offer a radical departure from traditional methods of construction and suggest ways to implement these design principles.

Curved concrete

By most measures, the world of concrete might at first seem a little boring. At a recent TED Talk however, University of Southern California professor Behrokh Khoshnevis unveiled his futuristic vision for 3D printing homes and buildings with a souped-up version of the stuff. If concrete can be successfully printed using his “contour crafting” method, the rectilinear grids found in most construction could be softened into strong, aesthetic curves. The formulation used would be a fiber composite with a compressive strength of 10,000 psi. By comparison, a typical batch of concrete used for homes, or the upper floors of tall buildings, has a strength of only around 3000 psi. It would also set fast enough to be fashioned on-the-fly into complex shapes, including arches and domes.

Most concrete is a tailored blend comprised of accelerants, retardants, plasticizers, bonding and pumping agents, corrosion inhibitors, and pigments. As anyone ever tasked with the job of chipping clean the traffic-hardened drum of a cement truck can tell you, the mixture has a narrow working window. Simply put: mixing and pumping concrete is not something you want to do on the mobile head of a three-story-tall printer.

To give an idea of what might be required to 3D print concrete, a pump-mixer that provides a steady flow through a 3-inch diameter hose at height of several stories would be powered by a V8 engine and is towed behind a decent-sized truck. Any printer like this would probably need a ground-based concrete supply and have a control system robust enough to handle the load of the feed mechanism. Current construction technology is already fairly sophisticated — with robotically-controlled boom trucks and pneumatically-applied “shot-crete” — but printing a habitable building in 20 hours would definitely be something new.

Silk-spinning spiderbots

The “spiderbot” that may some day handle large-scale 3D printing, is the brainchild of mechanical engineering student, Ben Peters. Peters is one of researchers in the Mediated Matter lab at MIT that focuses on bio-inspired 3D fabrication. Several of the lab’s projects aim to use silk-like materials as tensile elements or possibly as a base substrate on which other materials may be overlaid.

The spiderbot would essentially be a modified skycam — the kind of device you might see doing the filming a stadium event. A skycam can be pulled to and fro in the manner of a huge puppet, or alternatively it can have its own servo motors which traverse a support wire. The scale and speed of the skycam is impressive and using one to rough out suspensions for makeshift bridges or other constructions might be conceived. Fast-curing materials like expanding polyurethane foam could also be printed into molds with a spiderbot, later to be filled in a conventional manner with concrete. It would a be a stretch, but one might imagine a skycam repurposed for printing sideline markers, bases, or kitted out as an impromptu mini-Zamboni as the night’s events dictate.

Though it seems simple, researchers are at pains to mimic what a spider does reflexively. Up to eight spinnerets independently regulate the strength, adhesive properties, and elasticity of silk according to their needs. When they are done with their creations, spiders can consume them as food. It is not uncommon to see natural webs extending 15 or more feet across a clearing in the woods. Cunning arachnid architects do not typically create their first span by towing the silk line down one tree and up the next, but rather by turning out open-ended silk flow to the breeze and letting things take their course. In addition to its well known mechanical virtuosity, silk has many other useful properties (see: Dissolving electronics are a surprisingly simple answer to implantable tech) including excellent heat conduction and the ability to bear light like a fiber optic cable.

Application of the techniques of computer-aided manufacturing to these concrete- and silk-based materials — on any scale — will give great reward to those who might tame them. It may be a little while before we see silk-reinforced concrete fabs rolling down the highway on the back of a flatbed, but construction may be about to get a lot more interesting.

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